Warm water kills more than 1000 Kings in Blind Slough

An estimated 1,100 King Salmon died in Blind Slough on their way back to the Crystal Lake Hatchery to spawn. Low oxygen levels in the water, thanks largely to warmer temperatures, are to blame.

Increased water temperatures and low oxygen levels combined with decreased tidal activity in Blind Slough killed around 1,100 King salmon on their way to spawn at the Crystal Lake Hatchery two weeks ago.

A Fish and Game aerial survey taken a week before the salmon died revealed more than 1000 fish holding in deeper areas of Blind River Rapids. Alaska Department of Fish and Game Sport Fish Biologist Doug Fleming says he had been monitoring the salmon because of the warm summer weather. He discovered the fish carcasses Thursday, July 18.

Fleming says the incoming cold tidewater during the week only came in once and lasted for about an hour. He says this resulted in a “perfect storm” of conditions making water conditions unlivable for the salmon.

“The depth of the water was quite low,” Fleming said. “It was just barely enough to put some cooler water into the area where the fish were one time a day rather than two times a day.”

Warm water flowing from upstream made it difficult for the area to stay cool. And as water warms up, its ability to hold dissolved oxygen drops dramatically. At its peak, water in the Rapids reached 82.3 degrees Fahrenheit. Fleming says at those temperatures, water can only hold about 50 percent of its potential oxygen.

Like people, he adds, the already stressed fish probably couldn’t breathe and suffocated.

Fish and Game expected a return of around 1,800 adult King salmon to the hatchery. But they don’t project for younger fish, Jacks, which are one or two ocean-years-old.

“To say what portion right now was impacted, I wouldn’t go there,” Fleming says. “It’s a safe bet that it’s a stout impact. It’s probably on the order of 50 percent of that (projection).”

Roughly ten percent of Crystal Lake King salmon have coded wire tag codes. The data from those tags tells where the fish came from and what year it was released. Out of the 1,100 dead salmon, Fleming found 111 tagged fish. He says he won’t be able to gauge the overall impact until that data is sorted through.

The current fish kill will be added to a laundry list of bad returns. Fleming says cold winters, poor growth at the hatchery and low marine survival has all contributed to a decrease in Blind Slough salmon returns.

Some salmon stayed alive despite the harsh water conditions but those numbers are unknown.